Final answer:
The element most likely to form an ion with a -2 charge from the options provided is Oxygen, as it gains two electrons to form an oxide anion (O2-).
Step-by-step explanation:
The element that would most likely form an ion with a charge of -2 is Oxygen (A). This is because Oxygen commonly forms an oxide ion (O2-) by gaining two electrons to achieve a full outer shell, following the octet rule. In contrast, Sodium (B) commonly forms a cation with a charge of +1 by losing one electron. Chlorine (C) tends to form a chloride ion with a charge of -1 by gaining one electron. Lastly, Calcium (D) typically forms a cation with a charge of +2 by losing two electrons. Therefore, among the provided options, Oxygen is the correct answer.